The present invention concerns a hydraulic clearance compensating element for the intake and exhaust valve drives in an internal combustion engine. Such an element, or so-called "valve lifter", comprises two hollow cylinders mobilely sealed off from each other with caps over their opposite ends which engage each other axially by way of a compression spring.
A hydraulic valve lifter of this type is known, for example, from the German Patent No. 3,616,858. It is employed between the camshaft and the valves of an internal combustion engine to automatically adjust valve strokes. It is accordingly no longer necessary to adjust the valves during regular maintenance.
The known valve lifter has a piston that travels back and forth in a cylinder in the same direction in which the valve is actuated. The piston rests on a compression spring on the bottom of the cylinder. When there is no load on the valve lifter, the piston will accordingly travel in and out of the cylinder axially. Lubricating oil is drawn out of a reservoir through a constricted aperture and stored inside a pressurized compartment in the piston-and-cylinder mechanism. Since the constricted aperture is very small, the amount of oil stored in the piston-and-cylinder mechanism is not noticeably diminished during the brief interval that occurs while the valve is normally actuated by one of the cams on the camshaft. The forces that actuate the valve can be transmitted without any problem.
Sometimes, however, one of the cams will come to rest against a valve lifter while the engine is temporarily switched off, axially compressing it against the force of its interior compression spring. The oil in the pressurized compartment will increasingly leak back into the reservoir. The piston-and-cylinder mechanism can no longer take oil in rapidly enough, as the cam continues to rotate, to operate reliably at low temperatures.
For an internal combustion engine to function properly, the compression spring must reliably force the piston and cylinder apart rapidly enough even in cold weather to adjust the valve strokes. The spring must, of course, first force the oil in the reservoir into the piston-and-cylinder mechanism rapidly enough. Since oil becomes highly viscous at low temperatures, the spring must be very strong, and consequently those springs employed at the state of the art are very large. The unavoidable consequence is an undesirably strong force separating the piston from the cylinder at normal and especially at high temperatures, which can also contribute to undesirable wear and tear on the valve lifter.
A compression spring made of a shape memory metal alloy is known from "Legierung mit Formgedachtnis", Kontakt und Studium, Vol. 259, Expert Verlag, Ehningen. Depending on the composition of the alloy, the spring will support almost no load at a low temperature but will commence to support more and more load at a higher critical temperature or so-called "As temperature".